Dental implant system

Abstract: A dental implant assembly (10) is provided, as well as a system and method for exposing an embedded implant after osseointegration has taken place. The implant assembly (10) comprises an implant member (12) for embedding in the jaw and a rest factor member (18) for securing to the implant member, the rest factor member having an upper rest surface (42) just above the tissue level for opposing an overlying portion of a prosthesis (24) anchored elsewhere in the jaw to form a non-retentive rest or support for accepting down pressure from the prosthesis. The implant member (12) is relatively short and can be installed in distal jaw regions without interference with the mandibular nerve. A bore (14) is cut out in the jaw (16) for receiving the implant, inserting the implant and an attached healing screw (70) in the implant. The implant site is closed and osseointegration takes place over an extended period. Subsequently, the implant site is uncovered, the healing screw is removed, and the rest factor member (18) is secured in the implant.

Description: BACKGROUND OF THE INVENTION The present invention relates to a dental implant system. Dental implants are embedded in the jaw bone and serve to anchor one or more artificial teeth or dentures. Most implant systems involve a relatively long implant cylinder which is placed into a custom bored hole in the jawbone, then left for several months to allow healing and bone integration. Then the implant must be exposed for attachment of a dental prosthetic appliance such as a crown, denture, partial denture or bridge. This generally involves the dentist cutting out a flap of tissue which is peeled back to expose the implant, and secured by sutures after installing the prosthesis. This results in a relatively large area of trauma with a certain degree of pain to the patient and risk of post-operative infection. Another problem with conventional implants is their length, which makes them difficult to implant in the distal jaw region, where there is insufficient depth to enable their insertion without interference with the mandibular nerve, without the assistance of a dental surgeon to locate the precise position of the nerve and ensure that the implant does not interfere with it. A shorter cylindrical implant would not normally be suitable since it would provide insufficient "hold" and would likely become loosened with time if anchored to a denture or bridge. Also, side to side forces on the implant lead to bone erosion and trauma. Thus, dentures or bridges are often not anchored at the rear of the jaw. However, this has the disadvantage that trauma to the tissue and underlying bone beneath the denture occurs as a result of the denture repeatedly impacting the bone, particularly with long dentures which will tend to tilt or rotate about their attachment to anchor points during chewing or other jaw motions. This biting pressure can result in bone erosion or resorption down to the level of the nerve. EP-A-0288702 discloses a submersible screw-type dental implant including an implanted anchor portion and an abutment to support an artificial tooth. The anchor portion has a single lead thread with machined grooves, and a longitudinal central channel which directs bone chips towards the base of a bore in the patient's bone in which the implant is installed. SUMMARY OF THE INVENTION It is an object of this invention to provide an improved dental implant system and method which is less likely to cause significant tissue trauma and which reduces bone erosion as a result of denture wear. It is a further object of this invention to provide an improved system and method for exposing an implant site after osseointegration has taken place. According to a first aspect of the present invention, there is provided a dental implant assembly for embedding in the jawbone of patient, comprising: an implant member for embedding in and osseointegrating with the jawbone of a patient at a selected site; an attachment member for securing to the implant member after osseointegration; the implant member having an occlusal end portion with a central bore, the attachment member and central bore of the implant member having interengageable securing means for releasably securing the attachment member to the implant member, and a stem portion depending apically from the occlusal end portion for engagement with a recess in the jawbone of shape and dimensions matching those of the stem portion and, at is apical end, an annular indent; characterized in that the stem portion has outwardly projecting screw threads and the screw threads comprise multiple lead threads. The two members have cooperating releasable securing devices for releasably securing them together, preferably comprising a screw threaded bore in the implant member and a corresponding threaded portion on the attachment or rest factor member. The rest factor member projects up to just above the level of the tissue overlying the jawbone and has an upper surface opposing an overlying portion of a prosthesis anchored elsewhere in the jaw to form a rest for the prosthesis which accepts down pressure only, and which acts as a support to prevent or restrict bone erosion. The implant member has a selected height less than the depth of the mandibular nerve at the implant site, so that it can be embedded in the bone without risk of interference with the nerve. At the same time, the implant member is as wide as possible, and preferably has the maximum diameter possible according to the width of the patient's alveolar ridge at the implant site. The implant member diameter is preferably selected to be 1 mm less than the available alveolar ridge or bone width at the implant site. Implant members in a range of different heights and diameters are preferably provided to meet the requirements of a range of patients. Preferably, implant members with heights of 2 mm, 4.5 mm, 7 mm and 10 mm are provided, to allow for patients whose jawbone is already eroded to some extent. Implant members with diameters ranging from about 4 mm to 6 mm may be provided. Since the implant member is of relatively large diameter, it has a relatively large surface area resisting downward forces. In a preferred embodiment of the invention, the implant member has a generally cylindrical body with an occlusal end portion and apically depending stem portion which engages in a corresponding recess drilled out in the bone. Preferably, at least part of the stem portion at the apical end of the implant member has an annular recess forming an outer rim and central boss. This engages a corresponding annular recess drilled out in the bone to resist sideways movement of the implant. This will resist sideways movement of the implant during osseointegration, and also provides additional depth for securing the rest factor to the implant member. A bore of corresponding shape to the undersurface of the implant member is drilled out in the jawbone at the implant site, so that when the implant member is positioned in the bore, the peripheral rim will provide stabilization of the member against lateral movement during the osseointegration period. The shape of the undersurface of the implant provides a large area of bone to implant contact for osseointegration, and significant resistance to both lateral and downward forces both during and after the osseointegration period. Bone grows into the gaps between threads. The outer surface of the stem portion of the implant having the threads provides additional surface area for bone attachment. Bone grows into the area above and below the threads to resist loosening of the implant. Since the rest factor is not anchored to the prosthesis, the risk of jaw bone erosion or damage as a result of upward forces is reduced. However, the rest factor does accept down pressure as a result of biting pressure of the denture, and will thus reduce the risk of trauma to the tissue and jawbone erosion as a result of pressure. The localized contact between the rest factor and the underlying bone via the implant member reduces or substantially eliminates pressure trauma on the entire bone. If desired, the upper surface of the rest factor and the opposing portion of the prosthesis may be provided with opposing, non-retentive mating formations, such as opposing slightly convex and concave formations, for guiding the prosthesis against the rest factor. However, these formations do not provide any upwards retention of the prosthesis. A series of such rest factors may be provided at appropriate locations in the jaw where maximum down pressure from a denture is encountered, considerably reducing the discomfort of denture use and reducing the risk of tissue and jawbone damage as would result from conventionally anchored dentures. The rest factor may be preformed with a suitable rest surface in incremental heights, in which case the procedure after removing a healing screw comprises selecting an appropriate height rest factor and securing the selected rest factor in the implant member. The implant member may be relatively short with a relatively large diameter, so that it can be anchored securely in the jawbone without needing a deep bore to be drilled out. The implant member is provided in several heights. The shortest of the implants will be shorter and wider than conventional cylindrical implants, and thus can be used at the back or posterior mandible of the jaw where the nerve position prevents or restricts the use of long implants. This implant is particularly suitable for positioning a rest factor in the second molar area in conjunction with implant dentistry where cantilevered bridges or anterior implants need support or in other places where a rest factor is needed in dentistry. The implant requires less bone to be drilled out than conventional cylindrical implants, reducing or minimizing bone loss, and is able to accept hundreds of pounds of down pressure from an overlying denture or prosthesis. After bone integration, the implant can be recovered by piercing the tissue overlying a healing screw secured to the implant with a pointed end of a locating guide tool, probing the implant site with the pointed end until it engages a hole in the top of the healing screw, inserting the pointed end into the hole, utilizing a tissue punch centered on the guide tool to cut out a plug of tissue directly over the implant, and subsequently removing the healing screw. Thus, the locating tool and tissue punch can be removed together from the implant site, carrying with them the tissue plug to expose the healing screw for removal with a separate tool. The implant is then exposed for secondary healing or restoration procedures while a minimum amount of tissue has been disturbed and little or no suturing is required. This considerably reduces the trauma, secondary tissue healing, discomfort to the patient, and risk of infection. The implant member provides osseointegration with good resistance to loosening forces as a result of chewing. It may alternatively be used to secure other dental devices such as an implant denture anchor or an implant magnet abutment. An implant locating and exposing tool may be used to recover the implant. The tool comprises an elongate shaft having a head at one end and a locating probe at the opposite end having a sharp end for probing the tissue over an implant site to locate a central hole in the top of a healing screw, and a cutter member mounted on the elongate member with its cutting face facing in the same direction as the locating probe, the cutter member being movable along the elongate shaft to cut out a plug of tissue overlying a healing screw. Thus, the healing screw can be located and the overlying tissue removed in one step, without having to cut out a relatively large flap of tissue. The healing screw may be provided with a concave upper surface so that the locating probe will be guided towards the central opening. The implant assembly described above provides a rest surface for an overlying cantilevered bridge or denture which is anchored elsewhere in the jaw, on which the denture can rest and which accepts down pressure from the denture, reducing tissue trauma. Rest surfaces may be provided wherever needed, in conjunction with the conventional implants and anchors used for securing the denture or prosthesis in the jaw. The improved implant recovery tool and method produces minimal trauma when exposing a previously embedded implant for subsequent connection to either a rest factor or to a conventional anchor or magnet abutment. According to a second aspect of the present invention, there is provided an implant member for embedding in a patient's jawbone for attachment to other dental devices, the member comprising: a generally cylindrical body having an occlusal end portion for projecting above the jawbone and an apical stem portion for insertion in a recess prepared in a patient's jawbone; the occlusal end portion having a central bore for insertion of a portion of a dental device to be attached to the implant member; and the apical end portion having an outer cylindrical surface, characterized in that the apical end portion has a plurality of separate threads projecting outwardly from the outer surface, the threads comprising multiple lead threads. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of a preferred embodiment of the invention, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts, and in which: Figure 1 is a perspective view of a rest factor according to another embodiment of the invention; Figure 2 is a perspective view, partially cut away, of one example of an implant for the rest factor of Figure 1; Figure 3 is a side elevation view, partially cut away, of another implant with triple retaining threads; Figure 4 is a side elevation view of a further implant with quadruple threads; Figure 5 is a top plan view of Figure 2; Figure 6 is a jawbone section showing the addition of a shallow counter-bore; Figure 7 is a similar view showing the cutting of threads to receive an implant; Figure 8 is a similar view showing insertion of the implant; Figure 9 is a similar view showing attachment of the rest factor; Figure 10 is a side elevation view, partially cut away, of another modified implant having an alternative wrench element; and Figure 11 is a top plan view of the implant of Figure 10. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 illustrates a rest factor 250 according to the invention. The rest factor 250 is generally cylindrical and has an upper, dome-shaped portion 252 and a downwardly depending, threaded stem portion 254. The dome portion 252 has hex bore 256 formed in its upper end for gripping by a hex tool on installing or removing the rest factor 250, as will be explained in more detail below. The hex bore will be relatively small, preferably of the order of .050", and may be filled with a temporary sealant after installation. The rest factor 250 is made from a suitable metal such as titanium alloy. The dome portion has a wear resistant titanium nitride coating applied. Rest factors are provided in a range of different dimensions. The dome portion is preferably provided in a range of different heights, and in a preferred embodiment rest factors were provided with dome portions of height 1.5 mm, 2.5 mm and 3.5 mm for fitting patients with different gum thicknesses. The total height of the rest factor is preferably in the range 2 mm to 4.5 mm. Figures 2, 3 and 4 illustrate three alternative types of implant for implanting in the jaw bone at a selected site to receive rest factor 250 or other denture fitting devices such as denture anchors or magnetic abutments of a conventional type. The implant 260 of Figure 2 is relatively short and has a body of generally cylindrical shape having an upwardly tapering upper end portion 262 and a downwardly depending stem portion 264. A threaded bore 266 extends downwardly from the upper end of the implant for receiving the threaded stem portion of the rest factor 250. An eight-sided cut 268 extends through the threads, as illustrated in Figures 2 and 5. The outer diameter of the implant at its upper end 270 is dimensioned to match the outer diameter of the dome portion of the implant at its lower end 272. The stem portion 264 of the implant 260 has an annular recess 274 extending upwardly from its lower end to a position close to but offset downwardly from the lower end of the recess 268. Recess 274 forms an outer annular rim 276 and a central boss 278 at the lower end of the implant, and provides an area for bone growth and osseointegration upwardly into the implant. Thus, the implant is generally cup-shaped at its lower end, with a central protrusion in the cupped area. A triple lead screw having a first lead or thread 279, a second thread 280, and a third lead or thread 282 is formed around the outside of the stem portion 264. The threads 279,280,282 are preferably of square cross-section and project out a predetermined distance from the surface of the stem portion 264. The threads may each extend around approximately 180° and preferably do not overlap, although they may overlap in alternative embodiments. Figure 3 illustrates another implant 284 which is longer than that of Figure 2. Again, the implant is generally cylindrical and has a relatively thin upper annular portion 286 with a short, upwardly projecting and tapered rim 288 and a downwardly depending stem portion 290 of reduced diameter. A bore 292 projects downwardly from the upper end of the implant 284 and has a threaded portion 294 for receiving the threaded stem of a rest factor member. A ten-sided or decagon cut 296 is formed through the threads, and preferably extends about halfway down the bore 292. In this version, the bore 292 projects downwardly into the stem portion of the implant, since the upper portion 286 and rim 288 are shorter than those of implant 260. A triple lead thread having first, second and third threads 297,298 and 299 is formed around the outer surface of stem portion 290. Preferably, the threads 297,298 and 299 start at equally spaced intervals around the periphery of the stem portion adjacent its upper end but spaced downwardly a distance below the annular ring portion 286. The threads extend around the stem portion towards its lower end and terminate at a location spaced above the lower end of the stem portion. As in the version of Figure 2, the stem portion 290 has an annular recess 300 at its lower end forming an annular rim 302 and central boss 304. As in the previous embodiment, the upper end 306 of the implant is of diameter matching that of the lower end of the rest factor. Figure 4 illustrates another, longer implant 310 which is similar to that of Figure 3, apart from the fact that the stem portion 312 is longer and a quadruple lead screw is provided on the stem portion. The implant of Figure 4 is otherwise identical to that of Figure 3 and like reference numerals have been used for like parts. The implant has a bore identical to that of implant 284 extending downwardly from its upper end, and an annular recess at its lower end which is identical to the recess of implant 284. As in the previous two embodiments, the upper end 304 of the implant has a diameter matching that of the lower end of the dome portion of the rest factor. A series of four separate threads 314,316,318 and 320 is provided around the periphery of the stem portion 312. The threads start at equally spaced intervals around the periphery of the stem at a location close to but spaced slightly below the upper end of the stem portion, to leave a gap between the annular ring 286 and the threads. The gap is in the range from 1 mm to 2 mm. Although in the illustrated embodiment the implants 260 and 284 each have three threads, and the longest implant has four external threads, any one of these implants may have two, three or four threads. The double, triple or quadruple lead thread allows for quick and solid engagement into a tapped bone site. The projecting screw threads form an undercut region and bone growth and attachment in this area increases the strength of the attachment and resistance to loosening forces. The unique cup shaped recess design at the lower end of the implant is also designed to increase bone to implant surface contact and attachment area. The pitch or angle of the thread is preferably relatively steep and is preferably in the range from around 9° to 17°. The spacing between adjacent threads is preferably at least 1 mm. It has been found that this is the minimum spacing required to ensure significant bone growth and osseointegration in the gaps between threads. The threads preferably project out around ½ mm from the surface of the stem portion of the implant, and they are about ½ mm in height. Thus, the gaps between the threads are twice as wide as the threads themselves. This provides good blood supply to the bone between the threads, and provides a greater bone thickness and stronger shelf of bone between the threads. Both the implants and the rest factor are all made in a range of lengths and diameters to fit the size of different patient's jaws. Implants will be provided in a range of lengths. Preferably, the lengths are of the order of 2 mm, 4.5 mm, 7 mm, 10 mm and 13 mm. The 2 mm length implant will be of the type illustrated in Figure 2, while the 4.5 mm implant will be of the type illustrated in Figure 21 and the 7 mm, 10 mm and 13 mm implants will be as illustrated in Figure 4. Each of the different implants will be provided in a range of different outer diameters, preferably 4 mm, 5 mm and 6 mm. Preferably, the height of the tapered rim of the implant 260 or 310 is as short as possible, for example around 0.5 mm. The implants are made of a suitable material such as titanium and may be coated with a coating of a material for improving bone adhesion. The method of installing implants 260, 284 and 286 will be similar with appropriate changes in the dimensions of the drilled bore to accommodate the different length stem portions and numbers of threads. The method will therefore be described for the implant 260 only, by way of example. The first step in the implant procedure is to select the site in the jaw at which the implant is to be installed. The appropriate height and diameter implant member is selected dependent on the bone depth, tissue height, and jaw thickness at the selected site. The minimum bone depth required is 1 mm between the bottom end of the installed implant and the mandibular nerve canal or maxillary sinus space. Thus, the total bone depth required for installing the 2.5 mm implant is only 3.5 mm, while the longer implants can be installed where a greater bone depth is available. The minimum bone width required to place a rest factor implant is ½ mm each on the lingual and facial sides of the implant, requiring a 5.0 mm total ridge width for the 4.0 diameter implant, 6.0 mm ridge width for the 5.0 mm diameter implant, and 7.0 mm ridge width for the 6.0 mm implant. Based on these dimensions, the appropriate implant can be selected. The very short, 2.5 mm implant can be installed safely even where some bone erosion has already taken place and even in the distal jaw regions without risking interference with the mandibular nerve. The design is such that the implant has a relatively large surface area for bone to implant osseointegration, and undercut areas for improved retention and resistance to movement in any direction. Once the appropriate implant has been selected, a bore shaped and dimensioned to correspond to the shape and dimensions of the implant is cut out. This is done in a series of steps. First, a mesio-distal incision is made through the tissue or gum 321 along the alveolar mid-crest at the selected site, typically the area of the second molar. A bone plateau is created which is made as flat as possible by removing ridges or other bone irregularities. At this point, the width of the alveolar crest can be measured to determine the largest diameter rest factor implant which can be fitted within the available width and still allow at least ½ mm of bone on each side. The implant is placed as close as possible to the lingual side while still allowing the ½ mm of bone on the lingual side. At this point, a pilot burr is used to drill out a pilot hole of appropriate depth at the center of the insertion site. The pilot hole diameter will be substantially identical to the diameter of the implant central boss 278. An appropriate depth limiting sleeve is used with the pilot burr in order to limit the depth of the pilot hole to match the implant height. Thus, depth limiting sleeves will be provided for each of the four different implant heights, and the appropriate sleeve will be selected on installation. The lower end of a previously drilled pilot hole 322 in jawbone 323 can be seen in Figure 6, which illustrates a subsequent step in the procedure. An alignment pin may be placed in the pilot hole to check for proper alignment to the path of a prosthetic insertion and between multiple implants. After alignment, a spot-face drill (not illustrated) is used to make a shallow, 360° shoulder or seat 324 into the crest of the bone. This seat is used to assist in engaging of a bone tap in a later stage of the procedure. The drill is withdrawn several times during site preparation to remove bone buildup between the flutes of the drill. An implant body drill is used to drill out the main recess 328 for receiving the implant. This drill will have a diameter corresponding to that of the stem portion of the selected implant. Thus, the drill will have a diameter equal to the implant diameter less the thickness of the threads, or 4.0 mm in the case of a 5.0 mm implant, for example. In the next step of the procedure, a core drill 326 is used to cut an annular ring or recess 330 at the bottom of recess 328 for receiving the underside rim 272 of the implant. Drill 326 has a central guide 332 for centering in the previously drilled pilot hole 322, a cylindrical central body portion, and a downwardly facing annular ring of cutting teeth 334 for drilling out an annular groove or recess 330 for receiving the rim 272 (or 302 in the case of the implant 284 or 310) of the implant. A series of three guided hand taps (starting tap, intermediate tap and finish tap) are used to make a multiple lead tapped preparation into the drilled hole 328. It will be understood that the triple and quadruple leads will be made in a similar manner. One of the thread taps 336 is illustrated in Figure 7. Each tap has a tap guide pin 338 which engages the pilot bore to keep the tap centered on the recess. Optionally, a tap guide template (not illustrated) may be secured across the recess via pins secured on opposite sides of the implant site. This can be used to provide better support for the bone tap if the patient has spongy, cancellous bone. The tap 336 has triple lead cutting flutes 340 for forming a thread matching that on the insert. The starting tap has a machined line indicating the start of the double cutting edge of the bone tap. The starting tap is positioned into the drilled recess and the cutting flutes are engaged into the cortical bone, with the machined line on the tap positioned perpendicular to the longitudinal axis of the ridge The implant site is then hand tapped and threads 342, 343 and 345 are tapped into the recess. The tap is backed off a quarter of a turn several times during the process to clear bone chips. In this way, the tap is worked down to the bottom of the drilled recess 328. The procedure is then repeated using an intermediate tap, which will be marked with two lines to distinguish from the other two thread cutting taps. The lines are positioned in the same starting position as the line on the starting tap. Finally, the procedure is repeated with the finish tap, which will be marked with three lines, to create a final cut of the bone thread to accept the implant. An octagonal wrench 344 is used to place the implant into the prepared implant recess, as illustrated in Figure 8. The octagonal end of wrench 344 will engage in the octagon 268 cut through the threads 266 and will not damage the threads in bore 266. The implant is completely seated with the implant threads completely engaged in the bone threads and the tapered upper portion of the implant projecting above the level of the bone, as illustrated in Figure 8. If the implant is set in correctly, the stem portion 264 will have its upper end located ½ to 1 mm below the top of the bone, as illustrated in Figure 26. A healing screw is then threaded into the implant to protect the internal threads of the implant during the osseointegration period. The healing screw has a threaded stem matching bore 266 and a hex hole at the upper end of the screw. The flap of gum 321 is sewn down over the implant site after installation of the healing screw. The implant is then left for a healing period to allow the bone to osseointegrate with the surface of the implant. A healing period of not less than 4½ months should be used. The bone will grow over and osseointegrate into the gaps between the threads, providing a greater osseointegration surface area and better retention of the implant. The cup-shaped design of the lower end of the implant, along with the multiple lead threads, increases bone to surface contact and helps to withstand posterior occlusal load. Where a triple or quadruple lead thread is used, resistance to rocking or movement of the implant is provided. With two opposing threads, there may be some tendency for the implant to rock from side to side. Thus, implants with three or four threads are used for even greater resistance to rocking as a result of loads. After the minimum period required for osseointegration, an incision is made over the implant for access to the healing screw. Radiographic charts and finger palpation may be used to locate the general area of the implant. The incision may be made in a conventional manner, but preferably a guided tissue punch (not shown) is used. The tissue is pierced with the sharp point or probe of the tissue punch, and the implant site is probed until the point locates a hex hole provided on the top of the healing screw. The guided tissue punch is then turned down on the guide with firm pressure until it punches an opening through the tissue or gum. The tissue punch will engage the outside diameter of the healing screw, and will cut out a plug of tissue directly over the center of the implant. The locating tool/tissue punch assembly is then removed, simultaneously pulling out the cut plug of tissue. If the tissue does not pull out with the tissue punch, forceps may be used to remove it and expose the healing screw. A cover screw hex tool (not illustrated) is used to engage the hex hole at the top of the cover screw and remove the cover screw from the implant. The implant is now exposed for attachment of the dome-shaped rest factor 250 as illustrated in Figure 9. The implant may alternatively be attached to other dental devices, such as implant anchors or magnetic abutments for dentures. The dimensions of the selected rest factor will depend on the implant dimensions and also the tissue height of the patient. The height of the implant should be such that the top of the dome is at the tissue level or only slightly above it. The dome portion of the rest factor selected should therefore have a height substantially equal to the tissue or gum thickness. If the gingival layer has a height substantially greater than the largest dome portion, tissue reduction must be used to reduce the gingiva thickness to a maximum of 3 mm at the implant site. Once the correct size rest factor has been selected, a hex wrench 346 having a hexagonal shaft 348 is used to engage the hex bore 256 at the upper end of the dome-shaped upper end portion 252 of the rest factor. The rest factor is then threaded into the bore of the implant, as illustrated in Figure 9. The tissue is then sewn back around the implant dome, as indicated in dotted lines in Figure 9, and the dome is left in place for around 4-6 weeks to allow for a period of reduced implant loading during gingival healing. The patient's denture should be fitted to the dome top with a soft reline material to maintain proper occlusion during the gingival healing period. This is done by taking a full arch reline impression including the exposed metal tip of the rest followed to prepare a master cast of the jaw including the exposed tip of the dome. The denture base is relined using this cast in the normal manner. If a change in tissue height has occurred during the tissue healing period, the rest factor may be unthreaded and replaced with another rest factor of the correct dimensions. Once the healing period is complete, the top of the dome is exposed for a prosthesis to rest on with lateral freedom of movement. The preferred arrangement is to use the dome as a permanent, non-retentive rest when the patient has existing anterior retention for a prosthesis. However, the rest factor may be used as an attachment if required, for example if there is no existing retentive means for the prosthesis, or lateral stability is needed due to severely reduced ridge height. A hole is drilled into the center of the dome to an appropriate depth, and resin is injected into the counter-bore. When the resin starts to become firm, a post forming part of a standard rest plate of a denture is inserted into the bore until the bottom of the plate comes into contact with the resin. Figures 10 and 11 illustrate a modified insert 350 for use with a rest factor 250 in an equivalent manner to that described above. However, rather than having a multi-faceted cut for receiving a wrench in the threaded bore 352 of insert 350, an external, twelve-sided wrench element 354 is secured to the upper end of insert 350. Element 354 has an outer twelve-sided surface 356 for engaging a wrench with a corresponding internal bore, and a through bore 358 coaxial with threaded bore 352 but of larger diameter. Element 354 is preferably formed separately from insert 350 and then pressure bonded or welded to it. A conventional hex wrench may also be used to engage the element 354. Apart from threaded bore 352 and wrench element 354, insert 350 is otherwise identical to that of Figure 3 and like reference numerals have been used as appropriate. It will be understood that a wrench element 354 may also be provided on the insert of Figured 2 or Figure 4 in place of the multi-sided internal cut. This avoids the need for a precision machined cut extending through the threads, where the cut must be of precise dimensions between those of the inner and outer diameter of the threads to avoid damaging the threads. The use of an implant having two or more stabilizing threads on the outer surface of the stem embedded in the jawbone, combined with the inverted cup shape of the lower end of the implant, provides a stable implant which resists movement even in the case of the very short version. Bone growth into the inverted cup and between the threads provides a large area of bone to implant contact and osseointegration, providing significant resistance to both lateral and downward forces both during and after the osseointegration period. The non-retentive rest factor can be used to support dentures non-retentively in the posterior area of the jaw. As a result, the pressure applied to the lower jaw by the denture during mastication is dramatically reduced, reducing the discomfort and bone erosion normally resulting from denture wear. Although the implant can be installed anywhere in the jaw for retentive or non-retentive engagement with a denture or prosthesis, it is particularly useful for providing support in posterior regions of the jaw where the jawbone has become eroded. The accepted formula for cantilevered dentition attached to anterior implants limits the extension of the cantilever over the posterior ridge to no further than the distance between the plane of the most anterior implant and the furthest posterior implant. This results in a lack of support under the molar dentition and an extraction effect on the anterior implants during masticatory function. With the additional support provided by one or more rest factors and implants in posterior regions of the jaw, the extraction effect is significantly reduced or eliminated. In addition, trauma to the ridge under occlusal load, and the resulting bone loss, may be reduced. Although some preferred embodiments of the present invention have been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiments without departing from the scope of the invention, which is defined by the appended claims.

Claims: A dental implant assembly for embedding in the jawbone of patient, comprising: an implant member (260) embedding in and osseointegrating with the jawbone of a patient at a selected site; an attachment member (250) for securing to the implant member after osseointegration; the implant member (260) having an occlusal end portion (262) with a central bore (266), the attachment member (250) and central bore (266) of the implant member (260) having interengageable securing means for releasably securing the attachment member (250) to the implant member (260), and a stem portion (264) depending apically from the occlusal end portion (262) for engagement with a recess (330) in the jawbone of shape and dimensions matching those of the stem portion (264) and at its apical end, an annular indent (274); characterized in that the stem portion (264) has outwardly projecting screw threads (279,280,282) and, the screw threads (279,280,282) comprise multiple lead threads. The assembly as claimed in claim 1, wherein the threads (279,280,282) are of square cross-section. The assembly as claimed in claim 1 or 2, wherein the spacing between adjacent threads (279,280,282) is at least 1mm and the threads (279,280,282) run parallel to one another. The assembly as claimed in claim 3, wherein the gap between adjacent threads (279,280,282) is larger than the height of each individual thread. The assembly as claimed in any of claims 1 to 4, wherein the stem portion (264) has an occlusal end and an apical end, and the threads (279,280,282) start at a position spaced below the occlusal end of the stem portion (264). The assembly as claimed in claim 5, wherein the threads (279,280,282) terminate at a position spaced above the apical end of the stem portion (264). The assembly as claimed in any preceding claim, wherein the central bore (266) projects apically into the stem portion (264) of the implant member (260). The assembly as claimed in claim 1, including a wrench element (354) projecting from the occlusal end portion (288) of said insert member (350) for engagement with a tool for inserting the implant member, the wrench element (354) having a bore (358) aligned with the central bore (352) of said insert member (350). The assembly as claimed in any preceding claim, wherein said interengageable securing means comprises screw-threads, and said central bore (266) has a multisided cut (268) extending from the occlusal end of the bore (266) through at least part of the threaded bore for engaging a wrench. The assembly as claimed in any preceding claim, including a plurality of implant members (260) in a range of different heights from 2-13mm. The assembly as claimed in any preceding claim, wherein the attachment member (250) comprises a rest factor member having an occlusal rest surface for supporting, but not being connected to, overlying portions of a denture anchored elsewhere in the jaw, the rest factor member having an occlusal, dome-shaped portion (252) and an apically depending stem portion (254) for engagement in said implant member bore, the rest surface comprising the occlusal end of said dome-shaped portion (252). The assembly as claimed in claim 11, including a plurality of rest factor members having dome-shaped portions of different heights in the range from 1.5mm-3.5mm. The assembly as claimed in claim 11 or 12, wherein said dome-shaped portion (252) has a hex indent (256) at its occlusal end for engagement with a hex tool for insertion of the rest factor member into an implant member bore. An implant member (260) for embedding in a patient's jawbone for attachment to other dental devices, the member comprising: a generally cylindrical body having an occlusal end portion (262) for projecting above the jawbone and an apical stem portion (264) for insertion in a recess (330) prepared in a patient's jawbone; the occlusal end portion (262) having a central bore (266) for insertion of a portion of a dental device to be attached to the implant member; and the apical end portion (264) having an outer cylindrical surface, characterized in that the apical end portion (264) has a plurality of separate threads (279,280,282) projecting outwardly from the outer surface, the threads (279,280,282) comprising multiple lead threads. The implant member (284) as claimed in claim 14, wherein the occlusal end portion includes a cylindrical ring portion (286) of diameter greater than said cylindrical surface of the apical end portion (290), and an inwardly tapered rim (288) portion projecting occlusally from said ring portion (286). The implant member (284) as claimed in claim 15, wherein said ring portion (285) has a diameter equal to the diameter of said threads (279,280,282). The implant member (284) as claimed in claim 15, wherein said occlusal end portion as a height less than that of the apical end portion (290).

Inventors: ZUEST MAX, ZUEST, MAX
Applicants: ZEST ANCHORS INC, ZEST ANCHORS, INC.
Application year: 1994
Main classification: A61C8
IPC codes: A61C8